Wireline BOPs are typically included in the surface assembly at a wellhead when conducting well intervention services. The wireline BOP is typically installed below a grease injection head. In use, grease is pumped into the lower end of the grease injection head at some pressure above well bore pressure. Often, the grease is pumped into the grease injection head at 1.2 times well bore pressure.
By pumping grease into the grease injection head at a pressure above well bore pressure, the grease prevents well bore pressure from migrating past the lower end of the grease injection head. Additionally, because the grease surrounds the wireline that passes through the grease injection head, the grease prevents hydrocarbons from the well bore from flowing into and around the wireline and, thus, prevents migration of well fluids past the grease injection head.
While grease injection heads have proven very successful at holding back well bore pressure while conducting well intervention services, grease injection heads have sometimes failed to maintain an effective “grease seal” adequate to hold back well bore pressure. The grease seal can fail for any number of reasons, including the well experiencing a pressure spike while the intervention services are being performed and/or the grease thinning due to exposure to increased temperatures or well chemicals. On the rare occasions that a grease injection head fails to maintain an adequate grease seal, the rams of the wireline BOP are actuated to seal around the wireline and hold back well bore pressure.
Typically, a wireline BOP is equipped with multiple BOP rams that are assembled in a vertical stack that is positioned over and connected to the wellhead. The BOP has a central valve body with a vertical bore running through it. Wireline extends up through the center, vertical bore of the BOP stack. Depending on the operations being conducted on the well, other wellbore equipment may be within the vertical bore of the BOP stack at a particular time.
A typical wireline BOP has a plurality of laterally disposed, opposing actuator assemblies fastened to the valve body. Each actuator assembly includes a piston that is laterally moveable within an actuator body by pressurized hydraulic fluid (during normal operation) or by manual force (in the event of a failure of the hydraulic control system). Each piston has a stem threadably engaged or otherwise connected to it. The stem extends laterally toward the bore of the valve body and has a ram body attached to the end of the stem nearest the bore of the valve body.
Replaceable sealing elements are mounted within or on the ram bodies that extend into the vertical bore of the valve body of the BOP. When the pistons of the BOPs are moved to a closed position, commonly referred to as “closing the rams,” the vertical bore of the BOP is sealed and the well bore pressure is contained. The sealing elements mounted within or on the ram bodies are available in a variety of configurations designed to seal the vertical bore of the BOP valve body when the opposing rams and pistons are moved to their closed position.
Several types of ram and seal assemblies are used in the actuator assemblies of a BOP stack. A BOP stack typically includes one type of ram and seal assembly known as a “blind ram” that seals across the entire wellbore when no wireline (or other tubular) is located in the vertical bore at the location of the blind rams. The blind rams are designed to engage each other when the BOP is closed. Blind rams typically utilize seals with no opening in the face of the seals such that the blind rams form a complete seal through the vertical bore of the BOP.
Another type of ram and seal assembly, known as a “wireline ram” (or “pipe ram” for BOPs used with other types of wellbore tubulars) utilizes seals designed to seal around the wireline (or other wellbore tubulars) within the BOP's vertical bore. Like blind rams, the wireline rams are designed to engage each other when the BOP is closed. Each seal of a wireline ram, however, typically has a semicircular opening in its front face to form a seal around half of the outer periphery of the wireline. When the wireline rams are closed, the opposing wireline rams engage each other and seal the entire periphery of the wireline, thereby closing off the annulus between the wireline and the well bore surface.
A third type of ram known as a shear, or cutting, ram is designed to shear the wireline (or other tubular) when the shear rams are driven toward each other as the BOP is closed. In operation, the shear rams are typically used as a last resort measure to contain wellbore pressure from causing a blowout. A BOP with shear rams is typically the top section of a ram-type BOP stack, while various pipe rams and blind rams are typically located below the shear rams.
Should it become necessary to actuate a wireline BOP, it is desirable for the wireline rams to guide the wireline (or other tubular) to the center of the vertical bore of the BOP as the rams close to ensure that the ram bodies and/or the metal bodies of the inner seals do not damage the wireline within the bore. Given the high cost of wireline, centering the wireline to prevent damage or loss is very important. For example, if wireline is damaged or severed as the ram bodies close and seal around the wireline, the replacement cost of such wireline can amount to nearly $300,000 per 30,000 feet of wireline.
Prior art U.S. Pat. No. 6,676,103 (“the '103 patent”) discloses a guide system for centering wireline (or other tubular) in the vertical bore of the BOP when a set of wireline rams (or pipe rams) is closed. However, the guide system of the '103 patent has certain drawbacks, including the inability to replace the “guides” that guide the wireline (or other tubular) to the center of the vertical bore of the BOP without replacing the entire ram body, the distance between the guides and the seal assembly, and the potential for damaging the ram body in the event excessive “squeezing” force is imparted on the inner seal elements. Further, replacement of the sealing elements of such prior art pipe ram assemblies is more difficult, and requires extra expense, when compared to the inner guide seal assembly of the present invention.
The present invention offers an improved inner guide seal assembly that guides a wireline (or other tubular) to the center of the well bore and “energizes” the sealing elements of the assembly. The present invention helps prevent damage to the wireline due to misalignment and prevents the potential for excessive force building up in the inner seal elements such that they may deform or damage the ram bodies. Additionally, the inner guide seal assembly of the present invention allows for easy replacement of the entire seal assembly or, alternatively, replacement of the sealing element itself. Thus, the inner guide seal assembly of the present invention overcomes many of the drawbacks of the prior art.